function X = parseargs(X,varargin)

%PARSEARGS - Parses name-value pairs
%
% Behaves like setfield, but accepts multiple name-value pairs and provides
% some additional features:
% 1) If any field of X is an cell-array of strings, it can only be set to
%    one of those strings.  If no value is specified for that field, the
%    first string is selected.
% 2) Where the field is not empty, its data type cannot be changed
% 3) Where the field contains a scalar, its size cannot be changed.
%
% X = parseargs(X,name1,value1,name2,value2,...) 
%
% Intended for use as an argument parser for functions which multiple options.
% Example usage:
%
% function my_function(varargin)
%   X.StartValue = 0;
%   X.StopOnError = false;
%   X.SolverType = {'fixedstep','variablestep'};
%   X.OutputFile = 'out.txt';
%   X = parseargs(X,varargin{:});
%
% Then call (e.g.):
%
% my_function('OutputFile','out2.txt','SolverType','variablestep');

% The various #ok comments below are to stop MLint complaining about
% inefficient usage.  In all cases, the inefficient usage (of error, getfield, 
% setfield and find) is used to ensure compatibility with earlier versions
% of MATLAB.

remaining = nargin-1; % number of arguments other than X
count = 1;
fields = fieldnames(X);
modified = zeros(size(fields));
% Take input arguments two at a time until we run out.
while remaining>=2
    fieldname = varargin{count};
    fieldind = find(strcmp(fieldname,fields));
    if ~isempty(fieldind)
        oldvalue = getfield(X,fieldname); %#ok
        newvalue = varargin{count+1};
        if iscell(oldvalue)
            % Cell arrays must contain strings, and the new value must be
            % a string which appears in the list.
            if ~iscellstr(oldvalue)
                error(sprintf('All allowed values for "%s" must be strings',fieldname));  %#ok
            end
            if ~ischar(newvalue)
                error(sprintf('New value for "%s" must be a string',fieldname));  %#ok
            end
            if isempty(find(strcmp(oldvalue,newvalue))) %#ok
                error(sprintf('"%s" is not allowed for field "%s"',newvalue,fieldname));  %#ok
            end
        elseif ~isempty(oldvalue)
            % The caller isn't allowed to change the data type of a non-empty property,
            % and scalars must remain as scalars.
            if ~strcmp(class(oldvalue),class(newvalue))
                error(sprintf('Cannot change class of field "%s" from "%s" to "%s"',...
                    fieldname,class(oldvalue),class(newvalue))); %#ok
            elseif numel(oldvalue)==1 & numel(newvalue)~=1 %#ok
                error(sprintf('New value for "%s" must be a scalar',fieldname));  %#ok
            end
        end
        X = setfield(X,fieldname,newvalue); %#ok
        modified(fieldind) = 1;
    else
        error(['Not a valid field name: ' fieldname]);
    end
    remaining = remaining - 2;
    count = count + 2;
end
% Check that we had a value for every name.
if remaining~=0
    error('Odd number of arguments supplied.  Name-value pairs required');
end

% Now find cell arrays which were not modified by the above process, and select
% the first string.
notmodified = find(~modified);
for i=1:length(notmodified)
    fieldname = fields{notmodified(i)};
    oldvalue = getfield(X,fieldname); %#ok
    if iscell(oldvalue)
        if ~iscellstr(oldvalue)
            error(sprintf('All allowed values for "%s" must be strings',fieldname)); %#ok
        elseif isempty(oldvalue)
            error(sprintf('Empty cell array not allowed for field "%s"',fieldname)); %#ok
        end
        X = setfield(X,fieldname,oldvalue{1}); %#ok
    end
end
end
